An important development in oil and gas production in recent decades has been the drilling of horizontal legs of hydrocarbon wells in combination with improvements in hydraulic or other types of fracturing techniques for stimulating production from previously uneconomically tight formations. For some years, the fastest growing segment of gas production has been from shales or very silty zones that previously have not been considered economic. The current areas of increasing activity in the United States include the Barnett Shale, the Haynesville Shale, the Fayetteville Shale, the Marcellus Shale, the Eagle Ford Shale, the Bakken formation and other shale or shaley formations. There are similar formations in other parts of the world.
It is no exaggeration to say that the future of natural gas production and perhaps the future of oil production in the onshore United States is from heretofore uneconomically tight hydrocarbon bearing formations, many of which are shales or shaley silty zones. Accordingly, a development that reduces the cost of these type wells or increases cumulative production is welcome.
Currently, one procedure is to drill a horizontal leg through the productive formation, perform several frac jobs to generate vertical fractures at horizontally spaced locations along the horizontal leg of the well and produce the contents of the formation to the surface through conventional surface equipment. In order to frac a series of spaced locations in the horizontal leg, it may be necessary to set a bridge plug or other settable well tool to isolate the previously fraced zone from the next zone to be fraced. After all frac jobs are done, the settable well tools are removed, typically by drilling with a coiled tubing unit or with a work string and workover/completion rig.
There has been a trend to make bridge plugs and other drillable equipment from composite materials that can be more readily drilled than conventional cast iron. Thus, the only cast iron component of many currently available bridge plugs and other drillable downhole equipment is the slips that wedge the plug in the well. There has been a development of so-called “button” type slips that include a composite body having metal teeth embedded therein. These button slips are more easily drilled than conventional cast iron slips but there is a place for cast iron or other metal slips that are more easily drilled than current metal slips.
Cast iron metal slips are somewhat time consuming to drill up when one has the luxury of a workover rig working in a vertical well where drill collars can be used to apply weight to the bit. It is considerably more tedious to drill up a bridge plug using cast iron slips in a horizontal well or using a coiled tubing unit where very little weight can be applied to the bit.
Shales or other tight formations completed in a horizontal well section have a history of rapidly declining production so an economic limit is reached sooner than desired. One proposed technique to continue producing such a well is to refrac the well at intervals between the original fractures. This is currently accomplished by squeezing off the old fracture with cement, drilling out cement inside the casing string, reperforating the well between the old fractures and then refracing the well through the new perforations. The problem with squeezing off the old perforations is that one is never confident that the squeeze job won't fail at frac pressure in one or more of the perforations so frac fluid is diverted into an old fracture. If the original well had seven frac stages of four feet each with six perforations per foot, which is typical, there would be a total of a hundred sixty eight perforations to be squeezed. Expecting a squeeze job to hold over a hundred different perforations at frac pressures is a leap of faith.
It has been proposed to refrac old horizontal wells by setting a patch in the casing to cover the old perforations—a much more secure technique than squeezing with cement. After the casing patches are set, new perforations are sequentially created between the casing patches and the new perforations are sequentially fraced. In order to isolate a zone that has been fraced from the next zone to be fraced in such a well, a bridge plug or similar in tool is passed through the casing and casing patches to a location above the new perforations and then set against the casing. There are commercially available cast iron bridge plugs that are capable of passing through the reduced I.D. of a casing patch set in a casing string and then expanding into gripping and sealing engagement with the casing. Such cast iron bridge plugs are commercially available from all major oilfield service companies and have typically been used in vertical wells. In vertical wells, using a work over rig, a drill string and drill collars, enough weight can be put on the cast iron bridge plug to drill it up in a reasonable length of time. The problem is it is difficult and slow to drill up a single cast iron bridge plug in a horizontal well segment where very little weight can be applied to the bit. To date, it has not been possible to drill up two or more cast iron bridge plugs in a horizontal well because debris from the upper bridge plug interferes with and prevents drilling up the lower bridge plug due, in large measure, because not much weight can be put on the bit. In addition, it has not been possible to drill up two or more cast iron bridge plugs in a single bit run in a horizontal well because the effort completely wears out bits.
Cast iron slips are typically manufactured in one or two relatively large pieces. When the tool is assembled, two piece slips are held together in some fashion so they act as a one piece device. When the tool is set in the well, the slips are forced onto an expander cone which fractures the slips into a series of segments that are trapped between the expander cone and the inside of the casing. In the past, the slip segments are of one piece and extend along the axis of the well. When the tool is drilled up in order to conduct another operation, the expander cone is drilled up thereby freeing the slip segments. Because the slip segments are so large, they must be either ground up by the bit and circulated out of the well or allowed to fall into the rat hole below the lowest perforations in the case of a vertical well. In the case of a horizontal well, large slip segments must be reduced in size by the bit or mill in order to circulate to the surface.